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Arta and phase measurment
Arta and phase measurment
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Old 9th November 2017, 07:41 PM   #11
CharlieLaub is offline CharlieLaub  United States
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Originally Posted by kimmosto View Post
One problem with "single channel measurement + minimum phase extraction + acoustic center detection" -method is that measurement programs usually export responses with normal/measured phase - not the minimum. If polar response measurement set contains e.g. 108 measurements (3-way, hor plane only, steps 10 deg), it will take a while to export all one by one as minimum phase. Radically faster option would be exporting all with normal/measured phase and using batch converter. VituixCAD Calculator can convert all responses to minimum phase at once. After that user has MP responses, but he can't be sure how much acoustic centers have drifted in the off-axis data compared to reality, and how accurate minimum phase result is at the top octave because off-axis responses have directivity effects which make slope detection and estimation almost impossible.

Most of the work, obvious errors and remaining uncertainty will be avoided by using dual or semi-dual channel measurement and known (constant if possible) measurement distance from mic to baffle. That saves timing in measurement to all directions and with non-minimum phase systems.
Right, but that still does not help you if your measurement is only good to 100Hz and you need to extend that lower. Once you try to splice on other data (e.g. a nearfield measurement) you must recalculate the phase. That was the original motivation of my work, which was then generalized and extended.

Measured phase is just fine, however, time=0 is arbitrary, so I don't see a need for 2-channel measurements. As long as you keep the mic in the same exact location for all measurements taken at a particular angle to the "on-axis" position you have all the data you need. This is the case whether you do the extra processing and phase extraction (e.g. FRD Blender) or you just go with the measured phase.
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Old 9th November 2017, 08:20 PM   #12
kimmosto is offline kimmosto  Finland
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Originally Posted by CharlieLaub View Post
...I don't see a need for 2-channel measurements. As long as you keep the mic in the same exact location for all measurements taken at a particular angle to the "on-axis" position you have all the data you need.
Unfortunately not. For example ARTA normalizes single channel measurements so that highest IR peak starts at sample #300. Timing can drift depending on shape of IR curve. Off-axis measurements could have slightly different timing than axial because slope of IR is not so steep. Highest peak can also be from reflection with off-axis measurements to rear sector, especially with horns and other directive speakers. Measurement data can be totally useless mess.

REW normalizes single channel measurements same way for the display. Highest peak will be at 0 ms line -> timing depends on shape of frequency response. Fortunately REW can export also single channel responses with common time reference; moment when the signal is sent I suppose. That data could contain some latency variation, but I've never tested how much or little latency variations exists. (That alone does not make REW more valid tool for speaker engineering because few important features are still missing).

So, hardwired semi-dual is the first certain way to catch phase and timing with stable reference. That can also eliminate linear distortion of soundcard with ARTA (sweep signal only) assuming that channels are quite identical.
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Old 9th November 2017, 08:43 PM   #13
CharlieLaub is offline CharlieLaub  United States
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Originally Posted by kimmosto View Post
Unfortunately not. For example ARTA normalizes single channel measurements so that highest IR peak starts at sample #300. Timing can drift depending on shape of IR curve. Off-axis measurements could have slightly different timing than axial because slope of IR is not so steep. Highest peak can also be from reflection with off-axis measurements to rear sector, especially with horns and other directive speakers. Measurement data can be totally useless mess.

REW normalizes single channel measurements same way for the display. Highest peak will be at 0 ms line -> timing depends on shape of frequency response. Fortunately REW can export also single channel responses with common time reference; moment when the signal is sent I suppose. That data could contain some latency variation, but I've never tested how much or little latency variations exists. (That alone does not make REW more valid tool for speaker engineering because few important features are still missing).

So, hardwired semi-dual is the first certain way to catch phase and timing with stable reference. That can also eliminate linear distortion of soundcard with ARTA (sweep signal only) assuming that channels are quite identical.
Wow, I did not know that (!) about ARTA. Do you happen to know if that is documented in the ARTA manuals somewhere?

I probably did not realize that there was any time shift added because I almost exclusively make multiple measurements (single driver plus pairs of drivers) on each axis and then determine the offsets (pathlength differences) from acoustic center to mic for each. Yes, it does take a lot longer to do that!

While we are on the subject, what do you use as the second channel input? Do you split the input to the amp and route the same signal to the input used for the "second channel" in the 2-ch measurement?
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Old 9th November 2017, 09:52 PM   #14
kimmosto is offline kimmosto  Finland
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Originally Posted by CharlieLaub View Post
Do you happen to know if that is documented in the ARTA manuals somewhere?
I tried to find but no luck this time.

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Originally Posted by CharlieLaub View Post
While we are on the subject, what do you use as the second channel input?
I just loop line output 2 to mic/line input 2 and switch input 2 for line level signal. That is semi-dual, which is very safe compared to full dual with ICEpower etc. power amps which could explode with flashes

CLIO 11 is my main gear at the moment, but earlier it was ARTA (and some others).
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